The invention relates to rotary drill bits for use in drilling or coring deep holes in subsurface formations and, in particular, to a form of cutting element for use on such bits.
Rotary drill bits of the kind to which the invention relates comprise a bit body having a shank and an inner channel for supplying drilling fluid to the face of the bit. The bit body carries a plurality of so-called "preform" cutting elements. Each cutting element comprises a thin hard facing layer, which defines the front cutting face of the element, bonded to a less hard backing layer. For example, the hard facing layer may be formed of polycrystalline diamond or other superhard material, and the backing layer may be formed of cemented tungsten carbide. The two-layer arrangement of the cutting elements provides a degree of self-sharpening since, in use, the less hard backing layer wears away more easily than the harder cutting layer.
The preform cutting elements are usually mounted on the bit body by being bonded, for example, by brazing, to a carrier which may be in the form of a stud of tungsten carbide which is received and located in a socket in the bit body.
Examples of the use of such preform cutting elements, their manufacture and mounting on rotary drill bits are disclosed in U.S. Pat. Nos. 3,743,489, 3,745,623, 3,767,371, 4,098,362, 4,109,737 and 4,156,329.
Conventionally, the layers making up each cutting element are of uniform thickness, and the elements are most usually circular although other configurations are sometimes employed.
Although drill bits incorporating preform cutting elements of this kind are generally very effective, problems are often encountered through failure of the cutting elements by fracture or detachment from the bit when subjected to the very high stresses encountered during drilling. It is an object of the invention, therefore, to provide an improved form of cutting element which may be less susceptible to failure in use.